The shared epitope (SE) - a HLA-DRB1-encoded 5-amino acid sequence motif carried by the vast majority of rheumatoid arthritis (RA) patients - is associated with severe disease. The mechanistic basis of RA-SE association is unknown. In prior studies, this laboratory has demonstrated that the SE acts as a signal transduction ligand that activates nitric oxide (NO) production in other cells. SE signaling is transduced by cell surface calreticulin (CRT), a known innate immunity receptor previously implicated in immune regulation, autoimmunity and angiogenesis. To better understand the pathogenic role of the SE in RA, we have begun characterizing SE signaling events in RA-related cells. Preliminary data demonstrate that in dendritic cells (DCs) the SE is a potent inhibitor of regulatory T (Treg) cells and enhancer of pro-angiogenic, IL-17- producing T (Th17) cells. In endothelial cells (ECs), the SE triggers production of the pro-angiogenic factors, IL-8, IL-6 and monocyte chemotactic protein-1 (MCP-1). Additionally, it was found the CRT is over- citrullinated in the synovium of RA patients and citrullinated CRT showed markedly higher affinity to the SE and transduced much more potent signals than the unmodified protein. Thus, the SE triggers angiogenesis- relevant functional events that could account for its effect in RA. The main goal of the research proposed here is to characterize the effect of the SE in angiogenesis, a key pathgenetic mechanism in several autoimmune conditions, including RA. Specifically, we propose to: 1. Characterize the functional effect of SE-mediated Th17 polarization using mouse models of tolerance and collagen-induced arthritis (CIA); 2. Determine the effect of the SE-CRT on EC activation in experimental mouse models of angiogenesis and CIA; 3. Determine the role of CRT citrullination in SE-activated pro- angiogenic effect in mouse ECs Collectively, these studies offer an examination of a novel paradigm that provides a unifying explanation for several important mechanistic aspects of RA. In the long run, the new knowledge gained in this project could provide a basis for designing novel therapeutic strategies.